Optical apparatus using wavelength selective photocoupler

ABSTRACT

The present invention relates to optical apparatus such as a photosensor, a semiconductor laser, an optical amplifier in which a wavelength selective photocoupler is used so as to couple two waveguides through a diffraction grating. A photosensor which is one of the optical apparatus according to the present invention comprises a substrate, a first waveguide layer formed on the substrate, a second waveguide layer formed on the first waveguide layer to be stacked in a direction of thickness and which has a guided mode difference from that of the first waveguide layer, a diffraction grating formed on an overlapping region of the guided modes of the first and second waveguide layers and which couples light components of a specific wavelength range of light propagating through the first waveguide layer to the second waveguide layer, a light absorption layer for absorbing at least some light components of the light components coupled to the second waveguide layer, and an electrode for converting the light components absorbed by the light absorption layer into an electrical signal and outputting the electrical signal.

This application is a continuation of application Ser. No. 07/491,203filed Mar. 9, 1990.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical apparatus such as aphotosensor, a semiconductor laser, an optical amplifier, or the like,using a wavelength selective photocoupler which is constituted bycoupling two waveguides through a diffraction grating.

2. Related Background Art

A conventional wavelength selective photocoupler is constituted by twooptical waveguides formed on a single substrate, as described in, e.g.,Applied Physics Letters, R. C. Alferness, et al. 33, p. 161 (1978),Japanese Patent Laid-Open No. 61-250607, or Study Reports OQE81-129, theInstitute of Electronics and Communications Engineers of Japan. FIG. 1is a schematic perspective view showing a structure of a conventionalwavelength selective photocoupler.

In FIG. 1, two optical waveguides 81 and 82 are formed to have differentline widths, heights, and the like, and hence, have differentpropagation constants of guided light beams propagating along thecorresponding optical waveguides. In this case, a diffraction grating 83for compensating for a difference between the propagation constants isformed on one of regions where electric field distributions of the twoguided light beams are present, so that photocoupling between the twowaveguides occurs with respect to a guided light component in a specificwavelength range. More specifically, only a light component in aspecific wavelength range is selected to shift a light power between thewaveguides.

Such a conventional photocoupler is applied to a wavelength selectivefilter for combining/dividing signal light and a light wave of aspecific wavelength, and a photosensor for receiving a light wave of aspecific wavelength.

However, in the conventional photocoupler, since the waveguides areformed on a single plane, a difference between the propagation constantsof the two waveguides can only be controlled by line widths, heights,and the like of the waveguides, and a large propagation constantdifference cannot be obtained. For this reason, an optical power shiftoperation between the two waveguides is caused not only by thediffraction grating but also by an interference effect of two guidedlight beams. Therefore, it is difficult to obtain sharp wavelengthselectivity.

In the photocoupler described above, a direction of causing a powershift corresponds not to a direction of strongly confining guided light,i.e., a direction perpendicular to the substrate but to a planardirection with a relatively loose confinement effect. Therefore, acoupling length of a coupling region must be set as large as 3 to 15 mm.This coupling length makes the entire element large in size, anddisturbs the manufacture of an element utilizing waveguides formed of amaterial which cannot neglect absorption.

In order to further change the width and height of the waveguide, aphotolithographic technique is required. However, it is very difficultto attain precision of 1 μm or less in the manufacture, resulting inpoor reproducibility of elements and impairing element characteristics.

A wavelength selective photocoupler which can solve the above problemsis proposed in Integrated and Guided-Wave Optics, R. C. Alferness etal., 1989, technical digest series vol. 4, pp. 215-218. In thisphotocoupler, two waveguide layers of different guided modes are stackedon a substrate, and are optically coupled through a diffraction grating.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical apparatususing a photocoupler of a vertical structure, which has sharp wavelengthselectivity, and can decrease a light amount loss.

In order to achieve the above object of the present invention, there isprovided a photosensor comprising:

a substrate;

a first waveguide layer formed on the substrate;

a second waveguide layer formed on the first waveguide layer to bestacked in a direction of thickness, the second waveguide layer having aguided mode different from that of the first waveguide layer;

a diffraction grating formed on an overlapping region of the guidedmodes of the first and second waveguide layers, the diffraction gratingcoupling light components of a specific wavelength range of lightpropagating through the first waveguide layer to the second waveguidelayer;

a light absorption layer for absorbing at least some light components ofthe light components coupled to the second waveguide layer; and

an electrode for converting the light components absorbed by the lightabsorption layer into an electrical signal and outputting the electricalsignal.

In order to achieve the above object of the present invention, there isprovided a semiconductor laser comprising:

a substrate;

a first waveguide layer formed on the substrate and including a laseractive layer, the laser active layer emitting a laser beam uponinjection of a current;

an electrode for supplying a current to the laser active layer;

a second waveguide layer formed on the first waveguide layer to bestacked in a direction of thickness, the second waveguide layer having aguided mode different from that of the first waveguide layer; and

a diffraction grating formed on an overlapping region of the guidedmodes of the first and second waveguide layers, the diffraction gratingcoupling the laser beam emitted from the laser active layer to thesecond waveguide layer.

In order to achieve the above object of the present invention, there isprovided an optical amplifier comprising:

a substrate;

a first waveguide layer formed on the substrate;

a second waveguide layer formed on the first waveguide layer to bestacked in a direction of thickness, the second waveguide layer having aguided mode different from that of the first waveguide layer;

a diffraction grating formed on an overlapping region of the guidedmodes of the first and second waveguide layers, the diffraction gratingoptically coupling the first and second waveguide layers in a specificwavelength range;

a laser active region formed on at least a portion of the secondwaveguide layer, the laser active region amplifying light propagatingthrough the second waveguide layer upon supply of a current; and

an electrode for supplying a current to the laser active region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing a structure of aconventional wavelength selective photocoupler;

FIG. 2 is a schematic sectional view showing a basic structure, of awavelength selective photocoupler used in the present invention;

FIG. 3 is a graph showing light intensity distributions of guided modesof the photocoupler shown in FIG. 2;

FIG. 4 is a graph showing wavelength characteristics of light emergingfrom the photocoupler shown in FIG. 2;

FIGS. 5 and 6 are schematic views showing other structures of wavelengthselective photocouplers used in the present invention;

FIG. 7 is a schematic sectional view showing the first embodiment inwhich the present invention is applied to a photosensor;

FIG. 8 is a graph showing wavelength characteristics of light detectedby the photosensor shown in FIG. 7;

FIG. 9 is a schematic sectional view of the second embodiment in whichthe present invention is applied to a photosensor;

FIG. 10 is a graph showing wavelength characterstics of light detectedby the photosensor shown in FIG. 9;

FIG. 11 is a schematic sectional view of the third embodiment in whichthe present invention is applied to a photosensor;

FIG. 12 is a graph showing wavelength characteristics of light detectedby the photosensor shown in FIG. 11;

FIG. 13 is a schematic sectional view of the fourth embodiment in whichthe present invention is applied to a photosensor;

FIGS. 14, 15A, 15B are schematic views of the fifth embodiment in whichthe present invention is applied to photosensors;

FIG. 16 is a graph showing the relationship between a wavelength oflight detected by the photosensor of the fifth embodiment and a detectedcurrent ratio;

FIGS. 17 and 18 are respectively schematic views of the sixth andseventh embodiments in which the present invention is applied tophotosensors;

FIG. 19 is a graph showing the relationship between a wavelength oflight detected by the photosensor of the seventh embodiment and adetected current ratio;

FIGS. 20 to 22 are schematic views of the first to third embodiments inwhich the present invention is applied to semiconductor lasers;

FIGS. 23A and 23B are respectively a side view and a front sectionalview showing the first embodiment in which the present invention isapplied to an optical amplifier;

FIGS. 24A and 24B are respectively a side view and a front sectionalview showing the second embodiment in which the present invention isapplied to an optical amplifier; and

FIG. 25 is a schematic sectional view showing the third embodiment inwhich the present invention is applied to an optical amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a schematic sectional view showing a basic structure of awavelength selective photocoupler used in the present invention. In FIG.2, a 0.5-μm thick GaAs buffer layer 104, a 1.5-μm thick Al₀.5 Ga₀.5 Ascladding layer 105, a 0.2-μm thick first waveguide layer 101 in whichGaAs and Al₀.5 Ga₀.5 As layers are alternately stacked to constitute amulti-quantum well (MQW), a 0.7-μm thick Al₀.5 Ga₀.5 As cladding layer106, and a 0.45-μm thick second waveguide layer 102 in which GaAs andAl₀.4 Ga₀.6 As layers are alternately stacked to constitute an MQW aresequentially grown on a GaAs substrate 103 by a molecular beamexpitaxial (MBE) method. A diffraction grating 107 consisting of 0.2-μmdeep corrugations is formed on a portion of the upper surface of thesecond waveguide layer 102. The diffraction grating 107 is formed byusing reactive ion beam etching (RIBE) after a resist mask is formed. AnAl₀.5 Ga₀.5 As cladding layer 108 is regrown on the diffraction grating107 by a liquid-phase epitaxial (LPE) method.

In this manner, in FIG. 2, the waveguide layers (first and secondwaveguide layers 101 and 102) are stacked in a direction of thickness toconstitute a directive coupler. Since the waveguide layers 101 and 102are formed to have different thicknesses and compositions, light beamspropagating through these layers have different propagation constants.The diffraction grating 107 formed on the second waveguide layer 102 isused to select a wavelength to be optically coupled, and its pitchdetermines a wavelength range to be selected.

FIG. 3 shows photoelectric field distributions of guided modes of thephotocoupler shown in FIG. 2. A light intensity is plotted along theordinate, and a distance with reference to the first waveguide layer 101is plotted along the abscissa. In this embodiment, the guided modesinclude an odd mode 32 established to have the central intensity in thefirst waveguide layer 101, and an even mode 31 established to have thecentral intensity in the second waveguide layer 102. The diffractiongrating 107 is formed on an overlapping portion of the even and oddmodes 31 and 32.

The operation of the photocoupler shown in FIG. 2 having the twowaveguide layers will be described below.

Input light 109 consisting of a plurality of laser beam componentshaving wavelengths of 0.8 μm to 0.86 μm in units of 0.01 μm is input andcoupled to the first waveguide layer 101. The modes established in thetwo waveguide layers include the odd and even modes 31 and 32, describedabove. Light input to the first waveguide layer 101 is coupled to theodd mode 32 having the central intensity in the first waveguide layer101, and propagates therethrough. In this case, in a region without thediffraction grating 107, since the odd and even modes 32 and 31 havedifferent propagation constants, light beam components are not almostcoupled and propagate almost independently. However, in a region withthe diffraction grating, optical power shift occurs if the followingfunction is established between a propagation constant β₀ of the oddmode 32 and a propagation constant β₁ of the even mode 31: ##EQU1##where λ is the light wavelength, and Λ is the pitch of the diffractiongrating.

When the optical power shift described above occurs, guided light of theodd mode 32 coupled to the input light 109 is converted to guided lightof the even mode 31. Therefore, input light is finally converted to alight wave propagating through the second waveguide layer, and is outputas selected output light 110. Light beam components of other wavelengthsare directly output as non-selected output light 111. A region length lof the diffraction grating for causing perfect coupling is given by:##EQU2## where ε₁ and ε₀ respectively represent photoelectric fieldintensity distributions of the even and odd modes 31 and 32, and A₁ (x)is a component corresponding to 1st-order diffracted light of theFourier expansion of the diffraction grating.

In order to perform wavelength filtering to have light of a wavelengthof 0.83 μm as a central wavelength, Λ=9 μm from equation (1), and theperfect coupling length l=250 μm from equation (2).

As a result, of light components input to the first waveguide layer 101,the wavelength characteristics of the selected output light 110 outputfrom the second waveguide layer 102 are as shown in FIG. 4. As can beseen from FIG. 4, wavelength filtering of a full width at half maximumof about 170 Å is performed. The wave combination is also similarlyperformed, as a matter of course. Note that non-reflection coating isapplied to incident/exit end faces to reduce end face reflection.

FIG. 5 is a schematic perspective view showing another structure of aphotocoupler used in the present invention. FIG. 2 exemplifies a slabtype structure which does not perform lateral optical confinement.However, in FIG. 5, lateral optical confinement is performed to reduce alight loss. The same reference numerals in FIG. 5 denote the same partsas in FIG. 2, and a detailed description thereof will be omitted.

Referring to FIG. 5, in order to perform lateral optical confinement, animpurity such as Zn (or Si) is thermally diffused in the upper surfaceof the cladding layer 108 to disorder the two side portions of the firstand second waveguide layers 101 and 102, thereby forming low-refractionregions 141 having a low refractive index. Other structures are the sameas those in FIG. 2.

Guided light beams in the first and second waveguide layers 101 and 102are confined in the lateral direction by the low-refraction regions 141,and a loss caused by diffraction divergence of guided light can bereduced, thus obtaining a light wavelength filter having highefficiency.

Lateral confinement can be attained by various other methods, such as amethod of forming ridges, an embedding method, a loading method, and thelike.

FIG. 6 is a schematic sectional view showing still another structure ofa photocoupler used in the present invention. The same referencenumerals in FIG. 6 denote the same parts as in FIG. 2, and a detaileddescription thereof will be omitted.

In the structure shown in FIG. 2, the diffraction grating forcompensating for a propagation constant difference between the twowaveguide layers is formed in the second waveguide layer 102. In thestructure shown in FIG. 6, the diffraction grating is formed in acladding layer between the two waveguide layers. More specifically, asshown in FIG. 6, after an AlGaAs cladding layer 151 is formed, acorrugated diffraction grating is formed thereon by thephotolithographic method, and an AlGaAs second waveguide layer 152 andan AlGaAs cladding layer 108 are regrown thereon. Other structures arethe same as those in FIG. 2. The performance of the element in FIG. 6 isnot so different from that of FIG. 2. With these variations, a suitableelement shape can be selected when a photocoupler is combined withanother element or is formed as an integrated circuit. As describedabove, the formation position of the diffraction grating can bearbitrarily selected as long as both the photoelectric fielddistributions of guided light (the even and odd modes 31 and 32) arepresent there. In this case, coupling efficiency varies accordingly, anda coupling length must be adjusted.

FIG. 7 is a schematic sectional view showing the first embodiment of aphotosensor using the above-mentioned wavelength selective photocoupler.

The manufacturing process of this embodiment will be described belowwith reference to FIG. 7.

A 0.5-μm thick n-GaAs buffer layer 202, a 1.5-μm thick n-Al₀.5 Ga₀.5 Ascladding layer 203, a 0.2-μm thick n-Al₀.3 Ga₀.7 As first waveguidelayer 204, a 0.8-μm thick n-Al₀.5 Ga₀.5 As cladding layer 205, and a0.4-μm thick second waveguide layer 206 in which an MQW was constitutedby alternately stacking i-GaAs and Al₀.4 Ga₀.6 As layers weresequentially grown on an n⁺ -GaAs substrate 201 by the MBE method.Thereafter, a diffraction grating 207 consisting of corrugations eachhaving a depth of 0.05 μm and a pitch of 7.7 μm was formed on the uppersurface of the second waveguide layer 206 by the photolithographicmethod to have a length of 1.277 mm. An i-Al₀.5 Ga₀.5 As cladding layer208, and a 0.5-μm thick i-GaAs capping layer 215 were regrown on theresultant structure by the LPE method. Thereafter, the cladding layer208 and the second waveguide layer 206 in a region adjacent to thediffraction grating 207 were removed by etching. A 0.1-μm thick i-GaAsabsorption layer 209, a 1.2-μm thick p-Al₀.5 Ga₀.5 As cladding layer210, and a 0.5-μm thick p⁺ -GaAs capping layer 211 were regrown on theremoved portion by the LPE method. A Cr/Au electrode 212 was formed onthe capping layer 211, and an AuGe/Au electrode 213 was formed on therear surface of the substrate 201.

In the structure of this embodiment, of light 214 input to the firstwaveguide layer 204, only light components having wavelengths selectedby a light wavelength filter are coupled to the second waveguide layer206, and are absorbed by the absorption layer 209 serving as aphotosensor unit. The photosensor unit has a p-i-n structure, and areverse bias voltage is applied across the electrodes 212 and 213. Forthis reason, carriers produced by absorption are detected as currentsignals.

FIG. 8 is a graph showing wavelength characteristics of signal lineextracted as an electrical signal in the photosensor of this embodiment.

In this embodiment, since the depth of each corrugation is set to besmall, and a distance between the two waveguide layers is set to belarge, a large coupling length is attained. However, a wavelengthselection width is as small as about 30 Å.

FIG. 9 is a schematic sectional view showing the second embodiment inwhich the present invention is applied to a photosensor.

In this embodiment, a 0.5-μm thick n-GaAs buffer layer 305, a 1.5-μmthick n-Al₀.5 Ga₀.5 As cladding layer 306, a waveguide layer 301 inwhich 30-Å thick n-GaAs layers and 70-Å thick Al₀.5 Ga₀.5 As layers arealternately stacked to constitute a 0.2-μm thick MQW as a whole, a0.7-μm thick n-Al₀.5 Ga₀.5 As cladding layer 303, and a 0.4-μm thicki-GaAs light absorption layer 302 are sequentially grown on an n⁺ -GaAssubstrate 304 by the MBE method.

A diffraction grating 307 consisting of corrugations having a depth of0.2 μm and a pitch of 5.5 μm is formed on the upper surface of the lightabsorption layer 302 to have a length of 100 μm by etching using ammoniaand hydrogen peroxide after a resist mask is formed by thephotolithographic method using a photoresist.

A p-Al₀.5 Ga₀.5 As cladding layer 308 and a p⁺ -GaAs capping layer 309are grown on the resultant structure by the LPE method. An Au-Ge contactlayer (not shown), and an Au electrode 310 are formed on the rearsurface of the substrate 304. A Cr contact layer (not shown), and an Auelectrode 311 are formed on the upper surface of the capping layer 309.In this manner, a p-i-n type photodiode is formed.

With the above-mentioned structure of the photosensor of thisembodiment, the waveguide layer 301 and the light absorption layer 302stacked in a direction of thickness form a directive coupler. Thewaveguide layer 301 and the light absorption layer 302 have differentcompositions and layer thicknesses, so that propagation constants oflight beams propagating therethrough are different from each other. Thediffraction grating 307 formed on the upper surface of the lightabsorption layer 302 selects directly coupled light according to itsgrating pitch.

Photoelectric field intensity distributions of the even and odd modes 31and 32 established in the photosensor of this embodiment are the same asthose shown in FIG. 3. The waveguide layer 301 of this embodimentcorresponds to the above-mentioned first waveguide layer 101, and thelight absorption layer 302 corresponds to the above-mentioned secondwaveguide layer 102.

The operation of this embodiment will be described below.

A reverse bias voltage is kept applied across the electrodes 310 and311, and signal light 312 consisting of a plurality of laser beamcomponents of wavelengths 0.81 μm to 0.87 μm in units of 0.01 μm isincident on the waveguide layer 301 using end face coupling. The inputcoupled signal light 312 is converted to the odd mode, having thecentral intensity in the waveguide layer 301, of the even and odd modesshown in FIG. 3, and propagates through the layer 301. Since thephotoelectric field intensity distribution of the odd mode does notalmost influence the light absorption layer 302, a propagation losscaused by absorption is very small.

Since the even mode having the central intensity in the light absorptionlayer 302 has a propagation constant different from that of the oddmode, these two modes are not almost coupled to each other. However, ifthe relationship given by equation (1) is satisfied in a region with thediffraction grating, light of the odd mode is converted to that of theeven mode, and the central intensity is shifted to the light absorptionlayer 302. In this embodiment, the grating pitch Λ is set to be 5.5 μm,and a wavelength of 0.83 μm is detected. The guided light shifted to thelight absorption layer 302 is absorbed to cause electrons and holes tobe detected as a photoelectric current. FIG. 10 shows a wavelengthdistribution of detected light. As can be understood from FIG. 10, sharpwavelength selection of a full width at half maximum of 160 Å isperformed.

In this embodiment, the region of the diffraction grating is set to havea length of 100 μm which does not reach the perfect coupling length (thelength of a coupling region for maximizing coupling efficiency) of 262μm as a directive coupler using the diffraction grating. Such setup ismade in consideration of a response time of a photosensor. If anincrease in response time caused by an increase in light-receiving areais allowed, the length of the diffraction grating region is set to beapproximate to the perfect coupling length, thus further increasingabsorption efficiency.

If a plurality of elements according to the present invention areserially connected to have different pitches, an integrated photosensorcapable of simultaneously detecting signal light beams having aplurality of wavelengths can be manufactured.

FIG. 11 is a schematic perspective view showing a structure of the thirdembodiment of a photosensor according to the present invention.

In this embodiment, light detection is performed by a laterally formedp-i-n structure.

This embodiment was manufactured as follows. That is, a 0.5-μm thicki-GaAs buffer layer 402, a 1.5-μm thick i-Al₀.5 Ga₀.5 As cladding layer403, a 0.2-μm thick waveguide layer 404 in which 50-Å thick i-GaAs andAl₀.5 Ga₀.5 As layers were alternately stacked to constitute an MQW, a0.75-μm thick i-Al₀.5 Ga₀.5 As cladding layer 405, and a 0.3-μm thicki-GaAs light absorption layer 406 were sequentially grown on asemi-insulating GaAs substrate 401. A diffraction grating 407 consistingof 0.05-μm deep corrugations was formed on the upper surface of thelight absorption layer 406 following the same procedures as in thesecond embodiment shown in FIG. 9. The pitch of the corrugations was 4.6μm, and the length of the region was 200 μm. Thereafter, a 1.5-μm thicki-Al₀.5 Ga₀.5 As cladding layer 408 was grown on the resultantstructure, and an Si₃ N₄ protective layer 409 was formed thereon.

Zn and Si were thermally diffused in two side portions of the uppersurface of the protective layer 409 at an interval of 2 μm, therebyforming p- and n-type regions 410 and 413. Thereafter, a p⁺ -GaAscapping layer 411, and a Cr/Au electrode 412 were formed on the upperportion of the p-type region 410. An n⁺ -GaAs capping layer 414 and anAu-Ge/Au electrode 415 were formed on the upper portion of the n-typeregion 413.

Wavelength characteristics of a detection intensity with respect toinput light were observed in the same manner as in the second embodimentwhile a reverse bias voltage was applied to the lateral p-i-n structure.As a result, as shown in FIG. 12, good wavelength selectivity as in thesecond embodiment could be obtained. The full width at half maximum wasabout 33 Å.

Since the structure of this embodiment employs a semi-insulatingsubstrate, it allows easy electrical isolation from other elements, andis advantageous when a plurality of photosensors are integrated, or thisembodiment is integrated with a detection amplifier, a light-emittingelement, or a control driver.

FIG. 13 is a schematic perspective view showing a structure of thefourth embodiment of a photosensor according to the present invention.

This embodiment can provide an amplification function by an FETstructure as well as the wavelength division detection function.

The structure of this embodiment will be described below.

A 0.5-μm thick i-GaAs buffer layer 602, a 1.5-μm thick i-Al₀.5 Ga₀.5 Ascladding layer 603, a 0.2-μm thick waveguide layer 604 having the samestructure as that of the waveguide layer 404 in the third embodiment,and a 0.6-μm thick i-Al₀.5 Ga₀.5 As cladding layer 605 are sequentiallyformed on a semi-insulating GaAs substrate 601 by the MBE method as inthe third embodiment. A corrugated diffraction grating 607 is formed onthe upper surface of the resultant structure as in the second and thirdembodiments. A 0.4-μm thick n-GaAs light absorption layer 606 (dopingconcentration=1×10¹⁷ cm⁻³) is then regrown. Thereafter, a 0.3-μm thickSi₃ N₄ insulating film 609 is formed by sputtering.

As shown in FIG. 13, a source electrode 610, a gate electrode 611, and adrain electrode 612 are formed on the light absorption layer 606, thusconstituting an FET structure. The source and drain electrodes 610 and612 comprise Au electrodes having an Au-Ge layer as an underlying layer,and the gate electrode 611 is formed of Al.

The operation of this embodiment is the same as that of the aboveembodiment. Light incident on the waveguide layer 604 is mode-convertedin the diffraction grating region, and is absorbed by the lightabsorption layer 606. Carriers produced as a result of absorption areamplified, and are detected as drain currents.

In this embodiment, since the amplification function by the FETstructure is added to the wavelength detection function, a photosensorhaving high detection sensitivity can be obtained.

FIG. 14 is a schematic sectional view showing the structure of the fifthembodiment of a photosensor according to the present invention, and FIG.15A is a plan view showing the structure of an electrode formed on theupper surface of the fifth embodiment.

The manufacturing method of this embodiment will be described below.

A 0.5-μm thick GaAs buffer layer 704, a 1.5-μm thick Al₀.5 Ga₀.5 Ascladding layer 705, a 0.15-μm thick first waveguide layer 701 in whichGaAs and Al₀.4 Ga₀.6 As layers were alternately stacked to constitute anMQW, a 1.1-μm thick Al₀.5 Ga₀.5 As cladding layer 706, 0.2-μm thicksecond waveguide layer 702 in which GaAs and Al₀.2 Ga₀.8 As layers werealternately stacked to constitute an MQW, a 0.25-μm thick Al₀.4 Ga₀.6 Asdiffraction grating layer 707, and a 0.25-μm thick n-GaAs (n=1×10¹⁷cm⁻³) light detection layer 708 were sequentially grown on a GaAssubstrate 713 by the MBE method. A resist mask was formed on theresultant structure by the photolithographic method using a photoresist,and thereafter, the structure was subjected to selective etching usingan etchant mixture of ammonia and hydrogen peroxide solutions, therebyremoving the light detection layer 708 to leave a light detection region709 as a right-hand side portion in FIG. 14. In this case, since theAl₀.4 Ga₀.6 As diffraction grating layer 707 is present under the lightabsorption region 709, selective etching depending on properties of Alcan be easily performed.

A resist mask was formed again, and a ridge waveguide 901, extendingfrom the light detection region 709 to an introduction region 711, forconfining guided light was formed by RIBE, as shown in FIG. 15A. Adiffraction grating 902 having a pitch of 30 μm and a region length of550 μm was formed to be perpendicular to the longitudinal direction ofthe ridge waveguide 901 following the same procedures as for thewaveguide 901. The diffraction grating 902 is formed by etching theAl₀.4 Ga₀.6 As diffraction grating layer 707. Therefore, the depth ofthe diffraction grating is 0.25 μm.

A portion of the second waveguide layer 702 in front of the ridgewaveguide 901 was removed by etching so that only the first waveguidelayer 701 was coupled to light upon input of light (FIG. 14). In thiscase, a ridge end face 903 was obliquely etched with respect to thelongitudinal direction of the ridge waveguide 901, as shown in FIGS. 15Aand 15B to further reduce an opportunity of coupling to the secondwaveguide layer 702.

A 0.3-μm thick Si₃ N₄ film was formed on the upper surface of theresultant structure by plasma CVD (chemical vapor deposition). A resistmask for forming an electrode window in a portion of the light detectionregion 709 (FIG. 15A) was formed, and a portion of the Si₃ N₄ film wasremoved by dry etching using the mask. An Au/Au-Ge electrode film wasthen formed, and two electrodes 904 and 905 contacting the lightdetection layer 708 (FIG. 14) were formed by the photolithographicmethod, as shown in FIG. 15A. These two electrodes 904 and 905 arearranged to oppose each other to be perpendicular to the longitudinaldirection of the ridge waveguide 901, so that wavelength-selected guidedlight is coupled in the light detection layer 708 under a portionbetween the electrodes 904 and 905. The electrode length in a lightpropagation direction is 100 μm in this embodiment.

The length of a wavelength selection region 710 corresponds to that ofthe diffraction grating 902, and is 550 μm in this embodiment. Thelength of the light detection region 709 corresponds to that of theelectrode, and is 100 μm in this embodiment.

After the element prepared in the above process was cut, and the cutelement was fixed to a sample holder, and lead wires were bonded to theelectrodes 904 and 905 to be extracted to the outside.

In this manner, in the photosensor of this embodiment, the threewaveguide layers (the first waveguide layer 701, the second waveguidelayer 702, and the light detection layer 708) are stacked in thedirection of thickness, thereby constituting a directive coupler. Sincethe waveguide layers 701 and 702 and the light detection layer 708 areformed to have different thicknesses and compositions, light beamspropagating through these layers have different propagation constants.The diffraction grating 902 formed on the second waveguide layer 702 isused to select a wavelength to be optically coupled, and its pitchdetermines a wavelength range to be selected.

Since the light detection layer 708 is formed on the second waveguidelayer 702 in the light detection region 709, light of a selectedwavelength is absorbed, and carriers are produced accordingly. For thisreason, since a resistance between the electrodes 904 and 905 isdecreased according to the number of carriers produced byphotoconductivity, this decrease can be detected to detect light of theselected wavelength.

Note that TE₀ to TE₂ in FIG. 14 represent photoelectric field intensitydistributions in corresponding regions, and indicate light intensitiesin the propagation direction of guided light.

Since different semiconductor layers are stacked in the regions 709 to711 of this embodiment, different guided modes are established.

In the light introduction region 711, only a substrate mode, i.e., TE₀mode is established. In the wavelength selection region 710, an evenmode (TE₀) mode having the central intensity in the second waveguidelayer 702, and an odd mode (TE₁) mode having the central intensity inthe first waveguide layer 701 are established.

In the light detection region 709, two or more modes are establisheddepending on its multilayered structure. As shown in FIG. 14, TE₀, TE₁,and TE₂ are established. Guided light of each mode is absorbed by thelight detection layer, as described above. An absorption coefficient inthis case largely varies depending on the degree of influence of aphotoelectric field intensity distribution of each mode to the lightdetection layer 708.

The operation of this embodiment will be described below with referenceto FIGS. 14, 15A, and 15B under an assumption that a light signal 712incident on the first waveguide layer 701 includes light components of awavelength λ₁ selectively received by the diffraction grating 902, andlight components of a wavelength λ₂ to be cut.

The light signal 712 incident on the first waveguide layer 701 iscoupled to the TE₀ mode and guided in the light introduction region 711.In the wavelength selection region 710, light components of thewavelength λ₁ are selected by the diffraction grating 902, and arestrongly coupled to the TE₀ mode having the central intensity in thesecond waveguide layer 702, while light components of the wavelength λ₂are strongly coupled to the TE₁ mode having the central intensity in thefirst waveguide layer 701 and guided. In the light detection region 709,the light components of the wavelength λ₁ are strongly coupled to theTE₀ and TE₁ modes, and the light components of the wavelength λ₂ arestrongly coupled to the TE₂ mode. In this case, the light detectionlayer 708 absorbs light to produce carriers. Most of the lightcomponents absorbed in this case are of the TE₀ and TE₁ modes eachhaving the center of the photoelectric field intensity distribution inthe light detection layer 708 and a high absorption coefficient, i.e.,have the wavelength λ₁. As for the light components of the wavelengthλ₂, the center of their photoelectric field intensity distribution ispresent in the first waveguide layer 701, and does not almost cover thelight detection layer 708. Therefore, these light components are guidedwhile being coupled to the TE₂ mode having a low absorption coefficient,and are not almost absorbed.

FIG. 15B shows a circuit for detecting an amount of light absorbed bythe light detection layer 708.

A DC power source V1 is connected between the electrodes 904 and 905through a resistor R1 and an ammeter 906, and a change in suppliedcurrent value is measured by the ammeter 906, thus detecting a change inresistance across the electrodes 904 and 905. As a result, an amount oflight absorbed by the light detection layer 708 can be detected.

FIG. 16 shows a measurement result of a detected current ratio as afunction of wavelengths by introducing light components ranging fromwavelengths 0.80 μm to 0.86 μm as signal light 712 to the firstwaveguide layer 701. As shown in FIG. 16, the full width at half maximumat the central detection peak is 77 Å. Crosstalk between the wavelengths0.83 μm and 0.85 μm is about 20 dB.

FIG. 17 is a perspective view showing an outer appearance of the sixthembodiment of a photosensor according to the present invention.

In the fifth embodiment, light detection is performed by utilizingphotoconductivity. However, in this embodiment, light detection isperformed using electrodes having an FET structure. The manufacturingprocess and the structure are substantially the same as those in thefifth embodiment except for an electrode structure formed on the uppersurface of the structure.

The same reference numerals in FIG. 17 denote the same parts as in FIG.14, and a detailed description thereof will be omitted.

In the electrode structure of this embodiment, a third electrode isadded between the two electrodes arranged in the fifth embodiment.

More specifically, a total of three electrodes are arranged, as shown inFIG. 17, and serve as a drain electrode 801, a gate electrode 802, and asource electrode 803. The drain and source electrodes 801 and 803 areformed of Au/Au-Ge films, and the gate electrode 802 arrangedtherebetween is formed of an Al film.

The gate electrode 802 is in Schottky contact with the light detectionlayer 708, and a light detection layer immediately under the gateelectrode 802 serves as a depletion layer. When a negative voltage of -2to -5 V is applied to the gate electrode 802 while a positive voltage isapplied to the drain electrode 801 and a negative voltage is applied tothe source electrode 803, the depletion layer is widened, and a currentflowing in a source-drain path becomes 0 in a pinch-off state.

In this state, when guided light propagating from the wavelengthselection region 710 through the second waveguide layer 702 reaches thelight detection region 709, the guided light is absorbed by the lightdetection layer 708, thus producing carriers. Holes of the producedcarriers are absorbed by the gate electrode 802 to push up the depletionlayer. Therefore, a channel current flows through the drain-source path.In this case, electrons of the above-mentioned carriers become aphotocurrent flowing into the drain electrode 801.

More specifically, in a state wherein no guided light is incident on thelight detection layer, the source-drain path is in a pinch-off state,and no drain current is generated. However, when guided light isincident, a channel current caused by a change in depletion layer and aphotocurrent caused by production of carriers are generated. As aresult, a drain current is generated, thus detecting light.

In this case, when a gate voltage is controlled to be an appropriatevalue, crosstalk among wavelengths can be improved, and a detectedcurrent can be amplified. Unlike in the fifth embodiment, since thedetection operation is performed by using the FET structure, aphotosensor which allows a high-speed response of GHz or more can beobtained.

FIG. 18 shows the structure of the seventh embodiment of a photosensoraccording to the present invention.

In this embodiment, a PIN photodiode in which a light detection layercomprises an i-type layer as a light detection mechanism is formed.

The manufacturing process of this embodiment will be described below.

A 0.5-μm thick n-GaAs buffer layer 502, a 1.5-μm thick n-Al₀.5 Ga₀.5 Ascladding layer 503, a 0.1-μm thick n-Al₀.3 Ga₀.7 As first waveguidelayer 504, a 1.3-μm thick n-Al₀.5 Ga₀.5 As cladding layer 505, a 0.2-μmthick second waveguide layer 506 in which n-GaAs and n-Al₀.3 Ga₀.7 Aslayers were alternately stacked to constitute an MQW, a 0.15-μm thickn-Al₀.4 Ga₀.6 As diffraction grating layer 507, a 0.275-μm thick i-GaAslight detection layer 508, a 1.0-μm thick p-Al₀.5 Ga₀.5 As claddinglayer 509, and a 0.5-μm thick p⁺ -GaAs capping layer 510 weresequentially grown on an n⁺ -GaAs substrate 501 by the MBE method.Thereafter, a resist mask for protecting a light detection region 709was formed by the photolithographic method, and the p⁺ -GaAs cappinglayer 510 and the p-AlGaAs cladding layer 509 were etched by an etchantmixture of sulfuric acid, hydrogen peroxide solution, and water. Thei-GaAs light detection layer 508 was selectively etched using an etchantmixture of ammonia and hydrogen peroxide solutions, instead. In thiscase, since the underlying layer of the i-GaAs light detection layer 508is the n-Al₀.4 Ga₀.6 As diffraction grating layer 507 having an Alcontent of 40%, etching can be stopped at the i-GaAs layer.

Following the same procedures as in the fifth embodiment, a wavelengthselection region 710 was formed. The pitch of the diffraction gratingwas 22 μm, its length was 690 μm, and its depth was 0.15 μm. An Si₃ N₄protective film 511 was formed on the entire structure except for thelight detection layer 709.

Thereafter, an Au/Cr electrode 512 was formed on the capping layer 510,and an Au/Au-Ge electrode 513 was formed on the rear surface of thesubstrate 501.

In this embodiment, of signal light 514 input to the first waveguidelayer 504, only light components having a wavelength to be selected arecoupled the second waveguide layer 506 in the wavelength selectionregion 710. Thereafter, the light components are introduced into thelight detection region 709, and are absorbed by the i-GaAs lightdetection layer 508 on the second waveguide layer. The light detectionregion 70 has a p-i-n structure, and a reverse bias voltage is appliedacross the electrodes 512 and 513. For this reason, carriers produced byabsorption are detected as current signals.

FIG. 19 shows the relationship between signals extracted by thephotosensor of the seventh embodiment, and light wavelengths.

In this embodiment, since the depth of each corrugation is smaller and adistance between the waveguide layers is larger than those of the fifthembodiment, the coupling length is increased. However, the full width athalf maximum is as small as about 50 Å.

In the fifth to seventh embodiments, the light detection layer is formedand stacked on the second waveguide layer. Alternatively, the lightdetection layer may be formed beside the second waveguide layer, i.e.,to be parallel thereto on a single plane, thus achieving the object ofthe present invention. More specifically, the positional relationshipbetween the light detection layer and the waveguide layer is notparticularly limited as long as signal light is coupled from the secondwaveguide layer to the light detection layer, and is not coupled fromthe first waveguide layer to the light detection layer.

FIG. 20 is a schematic sectional view showing an embodiment in which thewavelength selective photocoupler described with reference to FIG. 2 isused in a semiconductor laser.

In this embodiment, a semiconductor laser unit 130 shown on theleft-hand side in FIG. 20 and a light isolator unit 125 illustrated inan embedded form on the right-hand side of FIG. 20 are formed on asingle semiconductor substrate 137. A laser beam emitted from thesemiconductor laser unit 130 propagates to the right of FIG. 20, andemerges outwardly through the light isolator unit 125. The semiconductorlaser unit is manufactured as follows. A GaAs buffer layer 138, anAlGaAs cladding layer 139, and a 0.3-μm thick first waveguide layer 124in which GaAs and AlGaAs layers were alternately stacked to constitutean MQW were sequentially grown on a GaAs substrate 137. An AlGaAscladding layer was grown on a portion, corresponding to thesemiconductor layer unit 130, of the upper surface of the waveguidelayer 124. The semiconductor laser unit 130 has its oscillationwavelength of 0.83 μm, and comprises a DBR (distributed feedbackreflector) region 122₁, an active region 121, a DBR region 122₂, and awaveguide region 136. A diffraction grating for constituting a DBR wasformed on a portion, corresponding to the DBR regions 122₁ and 122₂, ofthe upper surface of the waveguide layer 124. A portion, correspondingto the active region 121, of the waveguide layer 124 was formed as anactive layer 123, and GaAs capping layer 141 and an electrode 142 forinjecting a current into the active layer 123 were formed on the uppersurface of the cladding layer 140.

The structure of the light isolator unit 125 will be described below.

A 0.5-μm thick CdTe buffer layer 129, and a 1.0-μm thick Cd₀.4 Mn₀.6 Tesecond waveguide layer 131 were sequentially epitaxially grown on thewaveguide layer 124. A photoresist was applied on the upper surface ofthe waveguide layer 131, so that a photoresist mask having a pitch of9.8 μm was formed by mask exposure. Thereafter, two diffraction gratings132 each consisting of corrugations having a pitch of 9.8 μm were formedon two end portions of the upper surface of the waveguide layer 131 byRIBE. After the photoresist film was removed, a 0.3-μm thick CdTe firstcladding layer 133 was grown by epitaxial growth, and a Cd₀.4 Mn₀.6 Tesecond cladding layer 134 was then grown thereon.

With the above-mentioned process, the light isolator unit 125 in whichthe CdMnTe second waveguide layer 131 and the GaAs/AlGaAs firstwaveguide layer 124 were stacked was formed in a region adjacent to thesemiconductor laser.

In the two-layered waveguide, a guided mode established for a laseroscillation wavelength of 0.83 μm is a guided mode of a propagationconstant β₀ =3.322 having the central intensity in the waveguide layer124. In addition, a leaky mode of a propagation constant β₁ (=2.681)having a low refractive index and having the central intensity in thewaveguide layer 131 is available as a mode of a small propagation loss.Light 126 emitted from the semiconductor laser unit 130 is incident onthe light isolator unit 125 via the GaAs/AlGaAs waveguide layer 124, andis guided as guided light 127.

A propagation constant β_(LD) of this guided light 127 is 3.343, whichis very closer to the propagation constantβ₀ (=3.322) of the guided modeof the GaAs/AlGaAs waveguide layer in the light isolator unit, and thelight has a similar field distribution. Therefore, reflection andcoupling losses upon incidence of the light are very small.

After the light 127 is incident on the light isolator unit 125, it isguided while maintaining its guided mode. However, in a region with thediffraction grating 132, the light is oscillated by the diffractiongrating, and is coupled to other modes.

If a pitch Λ of each diffraction grating 132 is formed to compensate fora difference between the propagation constant β₁ of the leaky mode 128having the central intensity in the waveguide layer 131 and thepropagation constant β₀ of the guided mode having the central intensityin the waveguide layer 124 like in equation (1), these two modes arecoupled, thus causing optical power shift.

The diffraction grating 132 of this embodiment is formed to satisfyequation (1).

Since the above-mentioned coupling has a nature of wavelengthdivergence, a wavelength region satisfying equation (1) is limited.However, the wavelength bandwidth is normally several tens of Å toseveral hundreds of Å, and is sufficiently wide with respect to aspectral width, a variation, and instability of a laser oscillationbeam. Thus, no problem is posed.

When the semiconductor laser of this embodiment is used, an externalmagnetic field 135 having a predetermined magnitude and parallel to thepropagation direction of the guided light 127 is applied to the lightisolator unit 125. Thus, the external magnetic field 135 was applied tothe light isolator manufactured as described above to be parallel to thepropagation direction of light.

The operation of the light isolator formed as described above will bedescribed below. Light 126 emitted from the semiconductor laser unit 130propagates through the waveguide layer 124, and is incident on the lightisolator unit 125. After incidence, guided light 127 is shifted to thewaveguide layer 131 formed about the layer 124 in a region with thediffraction grating 132 to be converted to guided light 128 propagatingthrough the waveguide layer 131. The guided light 128 is shifted againto the waveguide layer 124 in a region with another diffraction grating132, and propagates therethrough.

The light 126 emitted from the semiconductor laser unit 130 isTE-polarized light. When the TE-polarized light propagates through thewaveguide layer 131 applied with the external magnetic field 135, itcauses a Faraday rotation of 45°. Therefore, the polarization directionof guided light emerging from the light isolator unit 125 is rotatedthrough 45° from the direction of the TE-polarized light. When thislight is reflected by various reflection surfaces of a coupling portionwith a fiber, a fiber connector, and the like, and returned, it returnsto the semiconductor laser unit 130 through the same optical path.However, since the light propagates the light isolator unit 125 in anopposite direction, it causes a Faraday rotation again, and its plane ofpolarization is further rotated through 45°. For this reason, the lightreturned to the semiconductor laser unit 130 is TM-polarized light. Thesemiconductor laser unit 130 which oscillates to emit TE-polarized lightis not easily influenced by return TM-polarized light. Thus, the elementof this embodiment can satisfactorily function as a light isolator.

In order to attain isolation having a large extinction ratio, a metalloading film may be added to the waveguide region 136 between thesemiconductor laser unit 130 and the light isolator unit 125, or thewaveguide region 136 may comprise a multilayered film of a metal and adielectric to form a polarizer which allows forward TE-polarized lightto pass therethrough but blocks backward TM-polarized light.

FIG. 21 shows the main part of the structure of the second embodiment ofa semiconductor laser.

In this embodiment, a diffraction grating 221 having a length twice theperfect coupling length with respect to guided light 127 is formed nearthe central portion of the upper surface of the waveguide layer 131 inplace of the diffraction grating 132 formed at two end portions of theupper surface of the waveguide layer 131 in the embodiment shown in FIG.20. Other structures are the same as those in the embodiment shown inFIG. 20, and the same reference numerals in FIG. 21 denote the sameparts as in FIG. 20.

In the structure shown in FIG. 20, after guided light 127 is coupled byone diffraction grating 132, it propagates through the waveguide layer131 by a predetermined distance, and is then returned again to thewaveguide layer 124 by the other diffraction grating 132. In thestructure of this embodiment, the diffraction grating 221 having alength twice the perfect coupling length is formed to obtain the sameeffect as described above. The guided light 127 continuously undergoespower shift to the waveguide layer 124→the waveguide layer 131→thewaveguide layer 124, thus obtaining the same Faraday rotation as in theembodiment shown in FIG. 20.

FIG. 22 shows the main part of the structure of the third embodiment ofa semiconductor laser.

In this embodiment, the structure of a light isolator unit is differentfrom that of the embodiment shown in FIG. 20, so that guided lightemerges from a waveguide layer formed in the light isolator unit. Themanufacturing process of this light isolator unit will be describedbelow.

A diffraction grating 231 consisting of corrugations having a pitch of16 μm was formed on the upper surface of the 0.3-μm thick waveguidelayer 124 described above by the photolithographic method and RIBE. A1-μm thick Cd₀.2 Mn₀.8 Te cladding layer 232, a 0.3-μm thick Cd₀.9 Mn₀.1Te waveguide layer 233, and a 1.5-μm thick Cd₀.2 Mn₀.8 Te cladding layer234 were sequentially formed on the resultant structure by the epitaxialgrowth method.

The operation of this embodiment is the same as that in the embodimentshown in FIG. 20, and a difference between a propagation constant β₀=3.320 of a mode propagating through the waveguide layer 124 and apropagation constant β₁ =2.809 of a mode propagating through thewaveguide layer 233 is compensated for by the diffraction grating 231having a pitch of 16 μm, thus causing optical power shift. As a result,guided light 127 emerges while its polarization direction is rotatedthrough 45° through the waveguide layer 233. In this embodiment, guidedlight directly emerges from the waveguide layer 233 without returning tothe waveguide layer 124. In this embodiment, the waveguide layer 233 hasa propagation mode surrounded by the cladding layers 232 and 234 havinga low refractive index, and a propagation loss is very small unlike inthe embodiment shown in FIG. 20.

FIG. 23A is a side sectional view showing an embodiment in which thewavelength selective photocoupler described with reference to FIG. 2 isused in an optical amplifier. FIG. 23B is a sectional view taken along aline A--A' of this optical amplifier. This embodiment is manufactured asfollows.

An n-GaAs buffer layer 2, a 1.5-μm thick n-Al₀.5 Ga₀.5 As first claddinglayer 3, a 0.2-μm thick waveguide layer 4 in which non-doped GaAs andAl₀.5 Ga₀.5 As layers were alternately stacked to constitute an MQW, ann-Al₀.5 Ga₀.5 As second cladding layer 5, and a 0.45-μm thick activelayer 6 in which non-doped GaAs and Al₀.4 Ga₀.6 As layers werealternately stacked to constitute an MQW were sequentially grown on ann-GaAs substrate 1 As the crystal growth method so far, a metal organicchemical vapor deposition (MO-CVD) method is used. However, the MBEmethod may be used. After the active layer 6 was formed, two diffractiongratings 9 and 15 suitable for wavelengths of signal light to beoptically amplified were formed on the upper surface of the active layer6 by photolithography to be separated at a predetermined distance. A1.5-μm thick p-Al₀.5 Ga₀.5 As third cladding layer 7, a 0.2-μm thick p⁺-GaAs capping layer 8, and an insulating layer 12 were sequentiallyformed on the upper portion of the resultant structure. A p-typeelectrode 13 was formed on a portion, corresponding to a portion betweenthe diffraction gratings 9 and 15, of the upper surface of the cappinglayer 8, and an n-type electrode 16 was formed on the rear surface ofthe substrate 1. The third cladding layer 7 and the capping layer 8 wereformed by the LPE method but may be formed by the MO-CVD method.

In order to form a three-dimensional structure of the waveguide layer 4,the two end portions of the waveguide layer 4 were removed by wetetching to expose the first cladding layer 3, as shown in FIG. 23B. Ap-Al₀.5 Ga₀.5 As buried layer 11, and an n-Al₀.5 Ga₀.5 As buried layer10 were grown on these removed portions to form a buried structure.

In this embodiment having the two-layered waveguide (the waveguide layer4 and the active layer 6), input light 14 on which a plurality of laserbeams of wavelengths 0.8 μm to 0.86 μm in units of 0.01 μm is input andcoupled to the waveguide layer 4. Modes established in the two-layeredwaveguide are the same as those described with reference to FIG. 3. Thewaveguide layer 4 of this embodiment corresponds to the first waveguidelayer 101 described above, and the active layer 6 corresponds to thesecond waveguide layer 102 described above.

The input light 14 is coupled to an odd mode 32 having a power peak inthe waveguide layer 4, and propagates through the waveguide layer 4. Ina region without the diffraction grating 9, since the odd mode 32 has adifferent propagation constant from that of an even mode 31, these modesare not almost coupled to each other (a maximum of about 1%), and almostindependently propagate through the corresponding layers. However, in aregion with the diffraction grating 9, if the relationship given byequation (1) is established between a propagation constant β₀ of the oddmode 32 and a propagation constant β₁ of the even mode 31, optical powershift occurs.

When the above-mentioned optical power shift occurs, signal lightincluded in guided light of the odd mode 32 coupled to the input light14 is converted to guided light of the even mode 31. Therefore, thesignal line propagates through the active layer 6. Light components ofother wavelengths in the signal light do not shift to the active layer6, and propagate through the waveguide layer 4.

A region length l for causing perfect coupling of each of thediffraction gratings 9 and 15 can be obtained by equation (2).

In order to perform wavelength filtering to have light of a wavelengthof 0.83 μm as a central wavelength, l=9 μm from equation (1), and theperfect coupling length l=250 μm from equation (2). The signal lineshifted to the active layer by the diffraction grating 9 is amplifiedduring propagation since the active layer 6 under the electrode 13serves as a laser amplifier unit having a gain. The amplified signallight propagating through the active layer 6 is coupled to the waveguidelayer 4 by the diffraction grating 15, as described above, and emergesfrom the end face of the layer 4.

In this manner, when current injection is performed only between thediffraction grating 9 and 15, a region of the active layer 6 excludingthis section serves as an absorption waveguide. Therefore, the amplifiedsignal light is never influenced by an unnecessary signal input, andnaturally emerging light having wavelengths other than a signalwavelength from the amplifier can be eliminated. The active layer 6 maybe removed by etching to leave a portion between the two diffractiongratings, thus obtaining the same effect as described above.

FIGS. 24A and 24B are views showing the structure of the secondembodiment of an optical amplifier.

In this embodiment, lateral optical confinement realized by the buriedlayers 10 and 11 in the embodiment shown in FIGS. 23A and 23B isachieved by removing two end portions of the structure by etching. Thestructure of this embodiment is substantially the same as that of theembodiment shown in FIGS. 23A and 23B, and the same reference numeralsin FIGS. 24A and 24B denote the same parts as in FIGS. 23A and 23B.

In the structure of this embodiment, after the third cladding layer 7was formed, its two end portions are etched to expose the secondcladding layer 5 (or the first cladding layer 3) by the reactive ionetching method (the RIBE or RIE method) using photolithography, thusforming a three-dimensional waveguide, as shown in FIGS. 24A and 24B. Ap-type impurity having a conductivity type opposite to the conductivitytype of the third cladding layer 7 was thermally diffused in the etchedend face to form an impurity diffusion layer 41. The same thermaldiffusion was performed in the input/output end faces of the activelayer 6, as shown in FIG. 24B, thus forming an impurity diffusion layer51.

The impurity diffusion layers 41 and 51 are formed to disorder the twoend portions of the active layer 6 and the waveguide layer 4. The reasonfor this will be described below. When the three-dimensional waveguidelike in this embodiment is formed, since a large number of interfacelevels are present in the two end portions of the active layer 6,injected carriers are recoupled through the interface levels, so thatthe number of insignificant injected carriers is increased, and signallight shifted from the waveguide layer 4 is undesirably absorbed

In the structure of this embodiment, the impurity diffusion layer 41disorders a superlattice structure in a direction perpendicular toguided light in the active layer 6 and the waveguide layer 4, and theimpurity diffusion layer 51 disorders a superlattice structure in adirection parallel to guided light in the active layer 6. Therefore, theactive layer 6 can be prevented from receiving unnecessary light, andnaturally emerging light other than a signal wavelength produced in anamplifier region can be scattered. It can also be prevented that thescattered light emerges together with the amplified signal light.

In the structure of this embodiment, the number of crystal growth stepscan be decreased as compared to the embodiment shown in FIGS. 23A and23B, and a waveguide width can be controlled by a thermal diffusiontime. Therefore, fine control can be realized.

FIG. 25 shows the structure of the third embodiment of an opticalamplifier.

In this embodiment, unlike in the first and second embodiments of theoptical amplifiers, a substrate 62, a buffer layer 63, a first claddinglayer 64, a waveguide layer 4, a second cladding layer 65, and an activelayer 6 were formed using non-impurity-doped GaAs and AlGaAs. Afterthese films were formed, a diffraction grating (not shown) was formed onthe active layer 6, and an n-type third cladding layer 66 and an n-typecapping layer 67 are regrown thereon. Thereafter, an Si₃ N₄ film wasformed as a diffusion mask, and 6-μm wide stripes were formed in themask by photolithography. Thus, the n-GaAs capping layer 67 wasselectively etched using the above mask and an etchant containingammonia and hydrogen peroxide solutions. ZnAs and a sample werevacuum-sealed on the capping layers 67 on the two ends of the structure,and were thermally diffused at 650° C. for 2.5 hours, thus forming animpurity diffusion layer. In this case, a diffusion front reached thefirst cladding layer 64, and both the waveguide and the active layer 6were disordered by Zn, thus forming a three-dimensional waveguide.Thereafter, the diffusion mask was removed, and the p-type layersdiffused in the capping layers 67 were removed by etching. After ap-type electrode 13 was formed, an insulating film (SiO₃) was formed.Through holes were formed in the insulating film by photolithography,and an n-type electrode 16 was formed through each through hole.

The element performance of this embodiment is not so different fromthose of the first and second embodiments of the optical amplifiers.However, in this embodiment, since layers up to the active layers 6comprise non-doped layers, the degree of freedom in element design canbe improved in the manufacture of an optical integrated circuit, and thelike.

In this manner, in the optical amplifiers of the first to thirdembodiments, since the two waveguides (the active layer and thewaveguide layer) are formed in a direction of thickness during crystalgrowth, an interval between the waveguides can be precisely determined.In addition, since the diffraction grating is commonly used, crosstalkbetween the waveguide layer receiving incident light and the activelayer can be reduced, and designs of crystal growth and the diffractiongrating are facilitated. As a result, a device can be easily optimized.

In the above embodiments, the input/output waveguide layer and theactive layer comprise the MQWs as the superlattice structure, but maycomprise conventional thin-film waveguides. The input/output waveguidelayer may be formed on the upper portion of the active layer. Theposition of the diffraction grating can be arbitrarily determined aslong as it falls within an overlapping region of the even and odd modes31 and 32 shown in FIG. 3. A large number of pairs of diffractiongratings having a plurality of different cycles may be arranged toperform light amplification for a plurality of wavelengths.

The present invention may be used in various applications in addition tothe above-mentioned embodiments. For example, each of the aboveembodiments comprises GaAs-based materials but may comprise III-V groupcompound semiconductors such as InGaAs, semiconductors such as Si, Ge,and the like, II-VI group compound semiconductors such as CdTe, and thelike in accordance with wavelengths to be detected.

The present invention incorporates all these applications within thespirit and scope of the appended claims.

What is claimed is:
 1. A semiconductor laser comprising:a substrate; alaser active layer formed on said substrate, said laser active layeremitting a laser beam by a current being supplied; a first waveguidelayer formed on said substrate, said first waveguide layer propagatingthe laser beam emitted from said laser active layer therethrough; anelectrode formed on said laser active layer, said electrode supplyingthe current to said laser active layer; a second waveguide layer formedon said first waveguide layer to be stacked in a direction of thickness,said second waveguide layer having a guided mode different from that ofsaid waveguide layer; and a diffraction grating formed on an overlappingregion of the guided modes of said first and second waveguide layers,said diffractions grating coupling the laser beam emitted from saidlaser active layer to said second waveguide layer.
 2. A semiconductorlaser according to claim 1, wherein said second waveguide layercomprises a light isolator for rotating a polarization direction oflight propagating through said second waveguide layer through 45° by aFaraday effect.
 3. A semiconductor laser according to claim 2, whereinsaid second waveguide layer essentially consists of CdMnTe.
 4. Asemiconductor laser according to claim 2, wherein said semiconductorlaser is divided into a laser unit and an isolator unit in thepropagation direction of light, and said diffraction grating is formedon only said isolator unit.
 5. A semiconductor laser according to claim4, wherein said second waveguide layer is formed on only said isolatorunit.
 6. A semiconductor laser according to claim 4, wherein said laserunit comprises a distributed feedback reflector.
 7. A semiconductorlaser according to claim 4 wherein two said diffraction gratings areformed to be separated by a predetermined distance in a propagationdirection of light, light coupled from said first waveguide layer andpropagating through said second waveguide layer being coupled to saidfirst waveguide layer again.
 8. A semiconductor laser according to claim2, further comprising first, second, and third cladding layers, saidfirst cladding layer, said first waveguide layer, said second claddinglayer, and said second waveguide layer being sequentially stacked in anorder named on said substrate.
 9. A semiconductor laser according toclaim 8, wherein said substrate, said first cladding layer, and saidfirst wave guide layer comprise a compound selected from the groupconsisting of GaAs and AlGaAs, and said second cladding layer, saidsecond waveguide layer, and said third cladding layer comprise acompound selected from the group consisting of CdTe and CdMnTe.
 10. Asemiconductor laser according to claim 1, wherein each of said first andsecond waveguide layers comprises a multi-quantum well structure.
 11. Asemiconductor laser according to claim 1, wherein said diffractiongrating comprises corrugations formed on said second waveguide layer.12. A semiconductor laser according to claim 1, wherein said diffractiongrating comprises corrugations formed on said first waveguide layer 13.A semiconductor laser according to claim 1, wherein said semiconductorlaser satisfies the following equation

    β.sub.1 (λ)-β.sub.0 (λ)=2π/Λ

where λ is a wavelength of guided light, β₀ (λ) is a propagationconstant of the guided mode of said first waveguide layer, β₁ (λ) is apropagation constant of the guided mode of said second waveguide layer,and Λ is a pitch of said diffraction grating.
 14. A semiconductor laseraccording to claim 13, wherein said semiconductor laser satisfies thefollowing equation: ##EQU3## where l is a region length of saiddiffraction grating, ε₀ is an electric field intensity distribution ofthe guided mode of said first waveguide layer, and ε₁ is an electricfield intensity distribution of the guided mode of said second waveguidelayer, and A₁ (x) is a component corresponding to 1st-order diffractedlight of the Fourier expansion of said diffraction grating.
 15. Anoptical amplifier comprising:a substrate; a first waveguide layer formedon said substrate; a second waveguide layer formed on said firstwaveguide layer to be stacked in a direction of thickness, said secondwaveguide layer having a guided mode different from that of said firstwaveguide layer; a diffraction grating formed on an overlapping regionof the guided modes of said first and second waveguide layers, saiddiffraction grating optically coupling said first and second waveguidelayers in a specific wavelength range; a laser active region formed onat least a portion of said second waveguide layer, said laser activeregion amplifying light propagating through said second waveguide layerupon supply of a current; and an electrode formed on said laser activeregion, said electrode supplying the current to said laser activeregion.
 16. An optical amplifier according to claim 15, wherein two saiddiffraction gratings are formed to be separated by a predetermineddistance in a propagation direction of light, light propagating throughsaid first waveguide layer being coupled to said second waveguide layerby one diffraction grating, and the light coupled by said secondwaveguide layer being coupled to said first waveguide layer again by theother diffraction grating.
 17. An optical amplifier according to claim16, wherein an impurity is diffused in a portion of said secondwaveguide layer located on a region excluding a portion between said twodiffraction gratings.
 18. An optical amplifier according to claim 15,further comprising first, second, and third cladding layers, said firstcladding layer, said first waveguide layer, said second cladding layer,and said second waveguide layer being sequentially stacked in an ordernamed on said substrate.
 19. An optical amplifier according to claim 18,wherein said substrate and said layers comprise a compound selected fromthe group consisting of GaAs and AlGaAs.
 20. An optical amplifieraccording to claim 19, each of said first and second waveguide layerscomprises a multi-quantum well structure.
 21. An optical amplifieraccording to claim 18, wherein said first cladding layer, said firstwaveguide layer, said second cladding layer, said second waveguidelayer, and said third cladding layer are mesa-etched excluding astripe-like region extending in a propagation direction of light, buriedlayers being formed on two sides of the mesa region.
 22. An opticalamplifier according to claim 18, wherein stripe-like ridges extending ina propagation direction of light are formed in said second claddinglayer, said second waveguide layer, and said third cladding layer byetching.
 23. An optical amplifier according to claim 15, wherein saiddiffraction grating comprises corrugations formed on said firstwaveguide layer.
 24. An optical amplifier according to claim 15, whereinsaid optical amplifier satisfies the following equation:

    β.sub.1 (λ)-β.sub.0 (λ)=2π/Λ

where λ is a wavelength of guided light, β₀ (λ) is a propagationconstant of the guided mode of said first waveguide layer, β₁ (λ) is apropagation constant of the guided mode of said second waveguide layer,and Λ is a pitch of said diffraction grating.
 25. An optical amplifieraccording to claim 24, wherein said optical amplifier satisfies thefollowing equation: ##EQU4## where l is a region length of saiddiffraction grating, ε₀ is an electric field intensity distribution ofthe guided mode of said first waveguide layer, and ε₁ is an electricfield intensity distribution of the guided mode of said second waveguidelayer, and A₁ (x) is a component corresponding to 1st-order diffractedlight of the Fourier expansion of said diffraction grating
 26. A lightamplifier comprising:a substrate; a first waveguide layer formed on saidsubstrate and having at least one end face, said first waveguide layerpropagating a light introduced from said end face; a second waveguidelayer formed on said first waveguide layer to be stacked in a directionof thickness, said second waveguide layer having at least one end faceat the same side of the end face of said first waveguide layer andhaving a different guided mode from that of said first waveguide layer;a diffraction grating formed on an overlapping region of the guidedmodes of said first and second waveguide layers, said diffractiongrating diffracting a light component having a specific wavelength rangeof light propagating through said first waveguide layer and convertingthe guide mode thereof to be coupled to said second waveguide layer; alaser active region formed on at least one part of said second waveguidelayer, said laser active region amplifying a light propagating throughsaid second waveguide layer by a current being supplied; an electrodeformed on said laser active region, said electrode supplying the currentto said laser active region; and means formed on said end face of saidsecond waveguide layer for preventing a light from being incident onsaid second waveguide layer.
 27. A light amplifier according to claim26, wherein said means for preventing a light from being incident onsaid second waveguide layer comprises an impurity diffusion layerprovided on an end face of said second waveguide layer.
 28. A lightamplifier according to claim 27, wherein said second waveguide layer hasa supper lattice structure, and said impurity diffusion layer is formedsuch that an impurity is caused to be diffused in said second waveguidelayer to make said super lattice structure disorder.
 29. A lightamplifier according to claim 26, wherein said diffraction gratingcomprises a first and a second diffraction gratings which are separateda predetermined distance in a light propagation direction, and a lightpropagating through said first waveguide layer is coupled to said secondwaveguide layer by said first diffraction grating, and the light coupledto said second waveguide layer and having propagated through said secondwaveguide layer is coupled to said first waveguide layer again by saidsecond diffraction grating.
 30. A light amplifier according to claim 26,wherein said diffraction grating comprises a currgation formed on saidsecond waveguide layer.
 31. A light amplifier according to claim 26,wherein he following equation is satisfied:

    β.sub.1 (λ)-β.sub.0 (λ)=2π/Λ

where λ is a wavelength of the guided light, β₁ (λ) is a propagationconstant of the guided wave mode of said second waveguide layer, β₀ (λ)is a propagation constant of the guided mode of said first waveguidelayer and Λ&0 is a pitch of said diffraction grating.
 32. A lightamplifier according to claim 31, wherein the following equations aresatisfied; ##EQU5## where l is a region length of said diffractiongrating, ε₀ is an electric field intensity distribution of the guidedmode of said first waveguide layer, ε₁ is an electric field intensitydistribution of the guided mode of said second waveguide layer and A₁(S) is a component corresponding to 1st-order diffracted light of theFourier expansion of said diffraction grating.
 33. A light amplifieraccording to claim 26 further comprising a first, a second and a thirdcladding layer, wherein lamination is made in success of said firstcladding layer, said first waveguide layer, said second cladding layer,said second waveguide layer, said third cladding layer and a lightabsorption layer on said substrate.
 34. A light amplifier according toclaim 33, wherein said substrate and each layer comprise GaAs or AlGaAs.35. A light amplifier according to claim 34, wherein said first andsecond waveguide layers have a multiquantum well structure.